Have you ever had the smooth vertical rods coming out while printing with your printrbot? I had that problem. When I got tired of constantly pushing them back in, I got to work designing this 100% printable, low-material, low-profile add-on to keep them in.
Print 2 of each stl file. Getting the plastic nuts on is a bit of a chore, but they don't slip like metal nuts do. The rest is explained in pictures.